1. Field of the Invention
The present invention relates to an image processing device, an image processing method, a program, a storage medium and an image processing system and more specifically to an image processing device, an image processing method, a program, a storage medium and an image processing system for performing image registration and difference processing between images.
2. Description of Related Art
The use of digital images in medical diagnostic imaging is rapidly increasing due to its advantages over conventional image radiographic systems. In digital radiography, a radiographic device using a semi-conductor image sensor is used to capture X-ray images. Such a radiographic device has the practical advantage of being able to record images in an extremely wide range of radiation exposure as compared with a conventional radiographic system using silver-halide films. Further, such a radiographic device has the practical advantage of being able to constitute an effective system in terms of storage and transmission of images.
Further, digitization of medical images has improved the diagnosis of such medical images. For example, when a conventional film radiographic system is used to read and compare X-ray images taken at different points of time for observation of the progress of a patient's disease, a radiologist generally views films on an X-ray film illuminator and performs comparative reading of the X-ray films.
When digitized image data is used, two digital images captured at different points of time are subjected to image registration so that corresponding locations in the two images are aligned with each other with respect to a normal anatomic structure. Subsequent to image registration, difference processing is performed to generate and output a difference image. A radiologist then reads and compares the difference image with the two original images so that changes occurring between the radiographs can be accurately determined.
A method for generating such a difference image is disclosed, for example, in U.S. Pat. No. 5,359,513, in which two chest X-ray images captured at different points of time are subjected to image registration and difference processing to generate a difference image. This difference processing is also known as temporal subtraction processing.
FIG. 6 is a block diagram showing a conventional image processing device for generating the above-described difference image. In FIG. 6, a first image and a second image indicate medical image data of a specific region captured at different points of time. A density correction portion 401 corrects the two image data such that the distribution of density values of image signals on one image becomes approximately equal to that on the other image. Then, an image registration portion 402 obtains and corrects the positional relationship between anatomic structures on the two images. A difference calculating portion 403 performs difference processing between corresponding pixels of the two images to generate a difference image. A display portion 405 displays the difference image along with the first and second images.
The two images are output to analog films or are displayed on a monitor for comparison with the difference image. Gradation characteristics of the medical images are also adjusted and matched with characteristics of the analog films. The disadvantage, however, is that it may be difficult to obtain gradation characteristics when a shadow portion is present in the medical image because the amount of signal corresponding to the shadow portion is small.
For example, in a chest X-ray image, an image of the lung field has high contrast, while an image of the mediastinal portion has low contrast. The shadow present in the image of the mediastinal portion provides a small amount of signal.
The above-described difference processing enables a difference image to show changes having occurred at two different points of time and, therefore, can improve the efficiency of comparative reading and prevent oversight of the changes. However, it is difficult to generate a difference image showing changes having occurred in an essentially low-contrast region, such as the mediastinal portion, in the same manner as in a high-contrast region, such as the lung field.
Moreover, in the case of comparative reading in lung disease, it is necessary to grasp average changes in size and density of a low-contrast shadow area having a certain degree of breadth, such as that caused by inflammation in the lung field. In conventional difference processing, however, if a change has occurred in a structure, such as a blood vessel, overlapping with such a shadow area, a difference image generated may also show a difference caused by that change. Therefore, some difference images may make it difficult for a radiologist to grasp changes in a specific shadow.